Alkaline solutions of hydrogen peroxide are commonly used for bleaching and/or delignification of cellulose pulp. The hydrogen peroxide is normally produced according to the anthraquinone process in large plants and transported to the pulp mills. The anthraquinone process is very effective, but requires high investments and is not suitable for small scale on-site production.
Electrochemical production of alkaline hydrogen peroxide solutions is disclosed in U.S. Pat. No. 5,702,585, "Process for the production of mixtures of caustic soda and hydrogen peroxide via the reduction of oxygen", P. C. Foller et al, Journal of Applied Electrochemistry, 25 (1995), p. 613-627 and in C. Oloman, "Electrochemical Processing for the Pulp and Paper Industry", The Electrochemical Consultancy 1996 p. 143-152. However, none of the documents disclose a process that is flexible to obtain various ratios NaOH.sup.- :H.sub.2 O.sub.2 in combination with production of one or more other chemicals that can be used at pulp mills.
Also chlorine dioxide is a commonly used bleaching agent at pulp mills. Due to the chemical instability of chlorine dioxide it is always produced on-site and many different processes are used commercially, such as those described in U.S. Pat. Nos. 5,770,171, 5,091,166, 5,091,167 and EP 612686. Most of the commercial processes involve reaction of sodium chlorate with a mineral acid, normally sulfuric acid, and a reducing agent, such as chloride ions, sulfur dioxide, methanol or hydrogen peroxide, in an acidic reaction medium. Sodium sulphate is normally obtained as a by-product, either as an acid or neutral solid salt cake or in the form of an acidic residual solution. In many modern pulp mills the sulphate by-product is considered as a useless waste material that has to be disposed, although there still is a demand for some of the sulphate for make up of the cooking liquor.
The formation of sulphate by-product can be decreased or completely avoided by replacing part of or all the sodium chlorate with chloric acid, as described in WO 93/25470. It is also possible to electrochemically acidify the solid sodium sulphate obtained, as described in U.S. Pat. No. 5,198,080, or to electrochemically acidify depleted reaction medium, without crystallisation, as described in U.S. Pat. No. 5,487,881. However, none of these methods have so far been commercialised.